This invention relates to a broadcast data receiver (BDR) and a method of use of said BDR.
The BDR referred to in the following description typically includes a display screen connected to or integrally formed with the BDR. The BDR allows for the generation of audio, visual and auxiliary data from digital data received from a broadcaster. In normal operation, a BDR receiving digital data will derive accurate frequency information from a stable reference embedded in the broadcast data stream.
BDRs typically contain a voltage controlled crystal oscillator (VCXO) generating a local frequency which is usually varied by up to +/−100 ppm about a nominal frequency, typically 27 MHz. Off air Moving Picture Expert Group MPEG transport stream/broadcast data streams have stable reference control signals embedded therein which are common to a 90 KHz system clock reference (SCR). The microprocessor of a BDR produces a suitable pulse width modulated (PWM) waveform based on the SCR value to control the frequency of the VCXO. An accurate VCXO frequency allows accurate color subcarrier frequency generation, thereby allowing the generation of the final video output.
Storage means such as low cost, high capacity hard disk drives are now being used for the recording and playback of audio and video data in BDRs. On occasions when the BDR is disconnected from the signal feed from the broadcaster, video data is read from the hard disk drive. Under these circumstances an accurate SCR is unavailable, as even though the stable reference signal can be recorded on the hard disk drive, upon playback it will have lost its accuracy. This is because it is being replayed under the control of the local VCXO frequency, which is not a stable reference, rather than the SCR frequency it was originally broadcast at.
Phase Alternation Line (PAL PAL decoders in televisions take composite video signals (i.e. video signals containing luminance, color and synchronization pulses which are generated by BDRs) from the video output of BDRs and recover the constituent red, green and blue components for display on the screen of the television or monitor. The PAL decoder in an average television can lock to a color subcarrier frequency generated by the VCXO deviating by up to +/−200 Hz. This equates to +/−45 ppm about the subcarrier frequency of 4.43361975 MHz. If the free run frequency of the 27 MHz VCXO deviates beyond this point, as it can do when an inaccurate Pulse Width Modulated PWM is generated due to the absence of a stable SCR value, it is possible for the PAL decoder to lose color lock. This can result in color loss to the image displayed on the display screen, which is undesirable.
A similar problem is encountered when playing back BDR recordings on a video cassette recorder (VCR), for example if the user is trying to clear hard disk space in the BDR by transferring data onto a VCR. VCR's are less tolerant to errors in color carrier frequency than television systems and can introduce further errors to the recorded SCR. As such, when the recording has been transferred from the BDR to the VCR and the VCR is then played back on the display screen of the television system, the error in the color sub-carrier frequency results in deviations in PWM greater than the PAL decoder of the television can lock onto. This results in color loss of the image displayed on the display screen.
The aim of the present invention is to provide a method for the production of a pseudo stable reference control for the reliable generation of composite video signals replayed from storage means using a BDR.